Traditionally, in environments such as underground mines, industrial structures, and commercial complexes where electromagnetic energy transfer is blocked or limited by the media or obstacles, communication has been achieved by hardwired systems such as telephone lines. The demand for higher workers' safety and for higher productivity stimulated product developments in an effort to use wireless two-way radios. Conventional wireless communication over long distances in these environments is not possible because the mine overburden (earth, water rock, etc.) and steel reinforced concrete in structures attenuate and dissipate the electromagnetic energy before it reaches its destination. Much of the development effort was directed to solve needs specific to the mining industry.
To meet mining communication requirements, it is necessary to have two-way communication between underground workers inside the mine and persons on the surfaces, and two-way communication between workers inside the mine and other workers inside the mine. In the mid 1970's the US Department of the Interior Bureau of Mines (USBM) engaged in efforts to develop wireless communication means for trapped miners. Collins Radio (USBM Contract No. HO133045) and General Instrument (USBM Contract No. JO35017) developed hand-held wireless transmitters that transmitted a tone (i.e., single frequency) that could be used by trapped miners to help surface rescue personnel to locate the trapped miners. Wide band (needed for voice or data) communication was considered impossible because of the amount of energy that would have to be in the transmitting antenna inside the mine that could cause a gas explosion in case of damage to the antenna.
USBM Report #RI9377 discusses and demonstrates the implementation of a method by which a wireless transmitter on the surface using a loop antenna could send a warning signal from the surface into the mine in case of an emergency, but no message could be sent from inside the mine to the surface. The transmitter antenna was fed from a 1 kW amplifier. The receiving antennas inside the mine were made of short high-permeability wound ferrite cores. Transmission from inside the mine to the surface was impossible.
Mine Site Technologies (www.minesite.com.au), and TeleMagnetic Signal Systems (NIOSH Report RI 9641-ISSN 1066-5552), developed one-way communication products to send warning signals to miners based on this USBM report. These systems are only capable of transmitting warning signals into the mine. Voice transmission is impossible in these systems. The signals transmitted from the surface to the interior of the mine cause the miner's cap lamp to flash. In the case of Mine Site Technologies, a warning message is displayed on an LCD. In the case of TeleMagnetic Signal Systems, the flashing lamp is a warning to evacuate the mine. If in addition a red LED is turned on, the miner must go to a telephone to receive the message.
U.S. Pat. No. 4,652,857 discloses a method and apparatus for a non-real time, two-way, wide-band communication for power restricted environments. The invention makes it possible to communicate voice or data from the surface of a mine into the mine, and from inside the mine to the surface. The method requires a loop antenna on the surface and a loop antenna inside the mine. The functioning of the system depends on magnetic Faraday coupling between the two antennas. To keep the antenna current inside the mine small enough to prevent an accidental explosion, the system trades power for bandwidth. This results in non-real time communication from inside the mine to the surface. For example, a six seconds message from inside the mine may take 60 seconds to reconstruct on the surface. Such operation is acceptable for emergency communication, but not for daily operational communication.
Several approaches have been advanced for communications inside a mine. A paper by Dobroski and Stolarczyk in IEEE Trans. on Industry Applications Vol. IA-21, July/August 1985 describes a method for transmission inside a mine using medium frequency. U.S. Pat. Nos. 4,879,755, 5,093,939, and 4,777,652 further teach the application of this concept. The methods taught by these patents depend on residual conductors that happen to be in the mine naturally or from previous structures. Signals “hop” from one conductor to another. Operation in arbitrary mines, therefore, cannot be guaranteed. The medium frequencies require the use of two-way radios that are larger than conventional two-way radios, and antennas that are so large that they must be carried by the miners either as shoulder straps which loop over a person's shoulder or they are sewn into a jacket that must be worn by the miner. Conspec/RIMtech, Raton Technologies, and others built products based on this method.
The most widely used method for wireless communications inside a mine uses so called Leaky feeders or radiating transmission lines. U.S. Pat. Nos. 5,432,838, 5,669,065, 5,697,067, and 5,809,429 discuss application of this method. Such a system makes use of transmission lines (also referred to as feeders or radiax antennas) that consist of coaxial cables with incomplete shielding so that electromagnetic energy is radiated from the cable along the length of the cable that can be received by mobile radios, and the antenna can receive signals transmitted from the mobile radios. The system also has a base station and amplifiers at spaced distances along the transmission lines. The lines are heavy, on the order of ⅝ inch in diameter. They are expensive and expensive to install and maintain. Since energy is gradually disseminated along the line and the total energy is restricted by FCC and safety regulations the energy at any distance from the line is relatively small. The radios between which communication is to be established must be in close proximity to the lines, requiring an extensive cable network throughout the mine. El-equip Inc., Tunnel Radio, Mine Radio Systems and others built products based on this method.
Therefore, it is highly desirable to have a system for providing a two-way bi-directional voice communications as well as digital communications in the environment where electromagnetic energy transfer is blocked or limited.